It is desirable to test bushings of the kind, for example, used in automotive vehicle suspensions for the purpose of quality control and, if possible, for testing during the process of developing a bushing or even during the development of the suspension system using the bushing.
The manner of fatigue failure of elastomers is an extremely complex phenomenon which is very difficult to predict due to the combination of physical and chemical deterioration. Several important factors affecting fatigue life are cyclic strain, prestress, environment, geometry, and composition. Failure begins when the material is unable to support local deformations. Flex cracks occur at points surrounding surface imperfections or filler boundaries, and grow upon repeated flexing.
In bushing testing, relatively large deflections place a demand on the test machine to physically constrain the test part in a manner closely approximating the actual end use. Improper constraint may drastically effect the performance of the specimen.
One approach to bushing testing is to test the sample using the entire end use assembly and real time loading conditions. This, however, presupposes the existence of a completed assembly and its availability for bushing test purposes. Road tests on an actual vehicle, if one exists with an appropriate suspension, are very useful but require that the vehicle be instrumented for the test and that a suitable road surface be available for the desired test.
The parallel development of the bushing and suspension often afford certain economic advantages. For this purpose the testing machine must be very flexible in terms of the kinds of input forces that can be applied and how well they can be controlled to simulate the forces arising from particular road surfaces and suspension designs.
The analysis of test results including statistical analysis is important in determining the physical properties of the specimen under test and to recognize the failure mode which may be defined as a change in spring rate, reduction in ultimate strength or complete sample failure, for example.
Testing machines for ball joints and the like with similar objectives have been proposed. In such machines inner and outer gimbals are rocked about their respective axes to afford the necessary imposition of torques on the test specimen. The outer gimbal carries the inner gimbal and also carries an actuator for driving the inner gimbal. The mass of the actuator causes the outer gimbal to be very much out of balance and increases the inertia of the outer gimbal so that its oscillation is limited to a frequency of about one Hertz, which is too low for many tests. In addition, hydraulic hoses attached to the actuator must withstand the constant flexing imposed by the oscillatory motion of the outer gimbal.
In addition to obviating the above problems with prior devices it is desirable to provide the ability to monitor rotary amplitudes of motion and load within 1% of their programmed amplitude. Since elastomers are temperature sensitive, it is also important to control the ambient temperature between 50.degree. and 120 degrees C. There is a conflict between the monitoring requirements and the temperature requirements since transducers should be coupled directly to the axis of motion to obtain the desired accuracy, but standard transducers which are economical to use generally do not operate in the required temperature range.